The invention relates to a cathode ray tube provided with a color picture screen, an electron gun for the emission of at least one electron ray, and a deflection device, which color picture screen comprises a front plate, a first set of detection strips and a second set of detection strips, a phosphor layer, a means for receiving signals generated by the detection strips, and a means for passing on correction signals to the deflection device.
A color cathode ray tube comprises a color picture screen, a neck, and a cone connecting the color picture screen to the neck, as well as an electron gun provided inside the neck for the emission of at least one electron ray. Deflection coils are present at the neck, deflecting the electron ray horizontally and vertically such that a raster of lines is created. In most cases, color cathode ray tubes have three electron guns for the three basic colors red, green, and blue, which are deflected as one whole by the deflection coils. The color picture screen has a phosphor layer in which the red-, green-, and blue-emitting phosphors are provided either as perpendicular strip triplets or in a pattern of dot triplets arranged in a triangle. To ensure that each of the three electron rays hits only the phosphors associated with it, a so-called shadow mask is provided immediately in front of the color picture screen. Each phosphor triplet is given exactly one opening in the shadow mask, for example a slot or a hole, through which three electron ray beams are passed at slightly different angles.
It is a disadvantage of such a shadow mask that almost 80% of the electrons do not pass through the shadow mask but hit the shadow mask and are subsequently removed. A further disadvantage of a shadow mask is that microphony effects may occur during operation. In addition, a shadow mask may become warped during operation owing to the heat generated therein, such that the openings of the shadow mask no longer occupy the correct positions.
A cathode ray tube with an arrangement for electron ray control is known from WO 00/38212 in which the position of the electron ray is determined in that conductor tracks are arranged under each phosphor strip. The position of the electron ray can be determined from the measured current difference between two adjoining conductor tracks, and a correction may be made, as necessary. Such cathode ray tubes are also denoted index tubes.
In this arrangement, the conductor tracks are covered wholly or partly by the phosphor layer. The phosphor layer itself is usually covered by a thin aluminum layer which is not in contact with the conductor tracks. The thin aluminum layer on the phosphor layer acts as a mirror and reflects light radiated inwards into the tube back in the direction of the front plate.
It is a disadvantage of this construction that only approximately 40 to 50% of the electrons emitted by the electron gun reach the conductor tracks. If the phosphor layer is not covered with an aluminum layer, approximately 70% of the electrons reach the conductor tracks. Owing to the low percentage of the electrons reaching the conductor tracks through the phosphor layer and generating a current signal there, the measured current signals are partly too small and insufficiently reproducible for enabling an unequivocal recognition and assignment of changes in the measured current difference at all times.
It is accordingly an object of the present invention to avoid the disadvantages of the prior art and to provide an improved cathode ray tube.
This object is achieved by means of a cathode ray tube provided with a color picture screen, an electron gun for the emission of at least one electron ray, and a deflection device, which color picture screen comprises a front plate, a first set of detection strips and a second set of detection strips, a phosphor layer, a means for receiving signals generated by the detection strips, and a means for passing on correction signals to the deflection device, wherein the detection strips each comprise a semiconductor component.
The use of a semiconductor component in each detection strip renders it possible to increase the current signals generated by the detection strips by several orders of magnitude. The measured current signal thus becomes sufficiently strong, also in the case of small electron currents, for changes in the measured current difference to be unequivocally recognized and for a decision on a position change of the electron ray to be possible. The enlargement of the measured current signal lowers the achievable black luminance and improves the dark contrast.
It is preferred that the semiconductor component is chosen from the group of pn diodes and Schottky diodes.
A pn diode is the simplest form of a semiconductor component and can be manufactured in a simple and inexpensive manner. The high-energy electrons of the electron ray are absorbed in the pn junction of the semiconductor component and produce a large number of charge carriers there. The charge carriers are separated in the field which is being generated and are measured as an amplified electron current.
In a Schottky diode, the high-energy electrons of the electron ray are absorbed in the depletion zone present below the metal in the semiconductor, where they produce a large number of charge carriers. The charge carriers are separated in the field which is being generated and are measured as an amplified electron current.
It is particularly preferred that the pn diode comprises a first electrode, an n-conductivity layer, a p-conductivity layer, and a second electrode.
It is particularly preferred that the p-conductivity region of the pn diode is arranged at the side which faces the electron gun.
It is furthermore preferred that the phosphor layer is covered with an aluminum layer.
The thin aluminum layer on the phosphor layer acts as a mirror and reflects visible light which is radiated inwards into the tube back in the direction of the front plate. The efficiency of the index tube is enhanced thereby.
It may be advantageous that it is true for the height H of a detection strip that H greater than D, with D being the layer thickness of the phosphor layer.
The arrangement of the detection strips between the individual phosphor strips of the phosphor layer prevents high capacitances from arising between the detection strips and the aluminum layer. These capacitances may adversely affect the time constant in the determination of the position of the electron ray. A further advantage of this arrangement is that no current compensation can take place between two detection strips through electron conduction effects in the phosphor layer.
It may be advantageous in this embodiment that the detection strips have a trapezoidal cross-sectional shape.
It may furthermore be advantageous in this embodiment that that side of a detection strip which adjoins the front plate is narrower than the side of the detection strip which faces the electron gun.
Thanks to this embodiment, the provision of the aluminum layer can take place by known vapor-deposition processes without the detection strips and the aluminum layer becoming electrically interconnected. Furthermore, the provision of the aluminum layer and of the second electrode of the semiconductor component with the pn junction may take place in one and the same step.
It may also be advantageous that the detection strips are covered by the phosphor layer.
It is preferred in this embodiment that the electrode of a pn diode lying at the side of the electron gun comprises a transparent material.
In this embodiment, the current signal is mainly generated by the photoelectric effect, i.e. through absorption of the light emitted by the phosphor in the pn diode. In addition, high-energy electrons penetrating the phosphor layer without appreciable energy losses will contribute to the current signal.
It is also preferred in this embodiment that a dielectric layer is provided between the phosphor layer and the detection strips and between the phosphor layer and the front plate.
If the refractive index of the dielectric layer is lower than the refractive index of the front plate, the quantity of specularly reflected light can be reduced by the application of the dielectric layer on the entire picture screen. This enhances the luminance of the cathode ray tube and/or prevents the occurrence of unpleasant external reflections. The dielectric layer also prevents short-circuits from arising during aluminizing of the color picture screen owing to aluminum penetrating through the phosphor layer to the electrodes of the semiconductor component, which would render the entire color picture screen useless.